New Techniques of Wavelength Measurements

Another method measures the absolute frequency of a stabilized laser and deduces the wavelength X from the relation X = c/v using the most recent experimental value of the speed of light [4.47]. Defining this value of c as the speed of light would allow the much higher accuracy of frequency measurements to apply directly to wavelength determinations [4.48]. This method will be discussed in Sect.6.10. 

For M = 100 this allows measurement of the ratio X /Xn with a precision 

Nr = MNqX /Xr, which is about l (8xl0 ) for our example above, provided the signal-to-noise ratio is larger than M. 

While the travelling Michel son wavemeter is restricted to cw lasers, a 

the wave number v can be deduced unambiguously with an accuracy determined by the interferometer with the highest path difference. The actual path differences as. are calibrated with a reference line and are servo-locked to this line. The precision obtained with this instrument is about 6 MHz, which is comparable with that of the travelling Michelson. The measuring time, however, is much smaller, since the different As. can be measured simultaneously. 

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